A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Optical element surfaces (e.g. in Extreme Ultraviolet (EUV) lithography apparatus) may suffer from contamination growth (e.g. carbonization) during apparatus operation. Such contamination may be introduced by the environment of the optical surfaces (e.g. vacuum environment, resist, radiation source, etc.). In an EUV lithography apparatus, multilayer mirrors typically provide these optical surfaces. To get rid of such contamination, cleaning of components is required. An example cleaning method is based on cleaning mirrors, in the optical path, with atomic hydrogen produced by, for example, a hot filament.